Recently, panel display apparatuses, such as liquid crystal display apparatuses, have become required to display various contents. Examples of contents displayed on panel display apparatuses include still images, 2D (2-dimensional) moving images and 3D (3-dimensional) moving images.
The inventors consider that, in displaying various contents thus described, it is appropriate to switch the frame rate (the number of frame images per unit time) in response to the contents. In the case that a liquid crystal display apparatus is used as a television, use of a frame rate corresponding to the double speed (120 Hz) successfully results in displaying images with reduced blur when contents consisting of still images or including many still images (for example, web contents) are displayed. For displaying a 2D moving image, on the other hand, it is preferable to use a higher frame rate, such as a frame rate corresponding to the triple speed (180 Hz) in order to reduce blur. For displaying a 3D moving image, in which left- and right-eye images are alternatively displayed, it is preferable to use a further higher frame rate, such as a frame rate corresponding to the quadruple speed (240 Hz). Use of a high frame rate independent of the contents may be acceptable if only the image quality is considered; however, the inventors consider that it is preferable that the frame rate is variable, instead of displaying images permanently with a high frame rate, since a high frame rate undesirably increases the power consumption of the display apparatus.
One issue the inventors have recognized in developing a display apparatus configured to vary the frame rate is that the necessary transmission rate of the display data widely varies depending on the frame rate. For example, the transmission rate of the display data for displaying images at the frame rate corresponding to the triple speed is one and a half times as much as that for displaying images at the frame rate corresponding to the double speed. The change in the transmission rate of the display data causes two problems. One is that the anti-EMI design of the printed circuit board and the transmission cables is made difficult. The changes in the transmission rate of the display data enlarge the frequency range of the EMI and this makes it difficult to take anti-EMI measures. Another problem is that, when a clock signal used in the data transmission is generated by a PLL (phase locked loop) or a DLL (delay locked loop), it is necessary to re-lock the PLL or DLL when the transmission rate of the display data is changed. The transmission of the display data is not allowed until the re-lock of the PLL or DLL is completed and this is not preferable for displaying images.
A disclosure of transmission of display data in a display apparatus is found, for example, in Japanese Patent Application Publication No. 2010-141775 A. This publication discloses a technique in which display data are stored in a frame memory after the display data are compressed and a display panel is driven in response to decompressed data obtained by decompressing the compressed data read from the frame memory. In this technique, the data compression ratio is adjusted depending on the display image. The data compression ratio is set high for reducing the power consumption for a display image which does not experience severe image quality deterioration even when the data compression ratio is large. For a display image which suffers from severe image quality deterioration when the data compression ratio is large, the data compression ratio is set low to maintain a sufficient image quality in displaying images in response to the decompressed data.
Also, Japanese patent gazette No. 4,507,265 B also discloses a technique in which display data are stored in a memory after the display data are compressed. This gazette discloses that the compression method is selected in response to the correlation among pixel data of four pixels.
The above-described techniques, however, do not address the changes in the necessary transmission rate of the display data, which are accompanied by the changes in the frame rate.